The present invention relates to the treatment of exhaust gases from diesel engines. More particularly, the present invention relates to the regeneration of catalysts that are used in the treatment of exhaust gases from diesel engines.
The primary harmful substances from diesel engines, apart from very small amounts of hydrocarbons (HC) and carbon monoxide (CO), are nitrogen oxides (NOx) and particles (PM). The particles are composed of both constituents that are soluble in organic solvents and constituents that are insoluble in organic solvents.
The soluble constituents consist of a number of different hydrocarbons that condense on or are absorbed by the core of the particles. The insoluble constituents are composed of sulfur trioxide or else sulfate, carbon, abraded metal (for example iron and nickel) and, in small amounts, other oxides as secondary products from additives in lubricating oils and in the fuel (for example, zinc, calcium and phosphorus). The sulfur trioxide may also be produced by oxidation of sulfur dioxide on the catalyst, depending on the temperature, noble metal loading and exhaust gas flow.
The ratio by weight of the various harmful substances with respect to each other in diesel exhaust gas depends on the type of diesel engine and its mode of operation. However, one common characteristic of diesel engine exhaust gas is the high oxygen content. Whereas the exhaust gas from stoichiometrically operated gasoline engines contains only about 0.7 vol. % oxygen, the exhaust gas from diesel engines may contain between about 6 vol. % and about 15 vol. % oxygen. In principle, the available data on exhaust gas applies to both stationary diesel engines and also to diesel engines in motor vehicles that are used in either light or heavy applications.
Governmental regulations and other legal standards govern the upper limits of permissible emissions from diesel engines. Different mechanisms may be employed in order to comply with these limitations. In part, the mechanisms selected depend on the type of diesel engine and its mode of operation. Two general tools that are well known to persons skilled in the art for addressing emissions from diesel engines are catalysts and soot filters.
One well-known catalyst is the diesel oxidation catalyst, which burns the hydrocarbons and carbon monoxide that are emitted by the engine, as well as some of the soluble organic compounds that are adsorbed on soot particles. For low power diesel engines in private cars, it is often sufficient to pass the exhaust gas over this diesel oxidation catalyst.
The oxidizing function of a diesel oxidation catalyst is designed in such a way that, although the organic compounds and carbon monoxide are oxidized, the nitrogen oxides and sulfur dioxide are not. The nitrogen oxides and sulfur dioxide emerge from the catalyst unchanged, together with the remaining fraction of the particles. A typical representative of these catalysts is described in DE 39 40 758 A1 (U.S. Pat. No. 5,157,007), which is incorporated by reference herein.
Soot filters are used in order to reduce the emission of soot particles, and are particularly useful in the case of diesel engines for heavy applications. The use of soot filters is well known by persons skilled in the art. In heavy applications, the exhaust gas back pressure increases continuously due to the deposition of soot particles on the filter. Consequently, the filter needs to be regenerated at regular intervals of time by burning off the soot. The soot emissions can be reduced by optimizing combustion in the engine. However, this automatically increases the emission of nitrogen oxides. These components will in turn need to be removed from the exhaust gas by catalysts such as HC-DeNOx catalysts and SCR catalysts.
HC-DeNOx catalysts are able to reduce the nitrogen oxides contained in the exhaust gas from a diesel engine to nitrogen, even in the presence of oxygen, using the hydrocarbons, HC, present in the exhaust gas. Under some circumstances, for this purpose, the concentration of unburnt hydrocarbons in the exhaust gas needs to be increased by appropriate measures.
Substantially more effective removal of the nitrogen oxides present in the exhaust gas from a diesel engine may be achieved by the process of selective catalytic reduction, using ammonia as a reducing agent. The reduction of nitrogen oxides takes place on SCR catalysts. The ammonia required is usually obtained from hydrolyzable compounds, such as for example, urea, with the aid of a hydrolysis catalyst.
The amounts of the exhaust gas, the composition of the exhaust gas and the temperature of the exhaust gas depend on the mode of operation of the vehicle, and thus, on the speed of the engine and the load. The development of diesel engines has led to a reduction in exhaust gas temperatures due to optimising the combustion process. The exhaust gas temperature of modern diesel engines, for example, in the partial load region, is only from about 80xc2x0 C. to about 150xc2x0 C. Only under full load does the exhaust gas temperature rise to more than 450xc2x0 C.
It was observed by the inventors that when treating the exhaust gases from modern diesel engines with the aid of a diesel engine oxidation catalyst, the catalytic activity decreased more rapidly than expected, although no thermal damage to the catalytic layer could be present due to the low exhaust gas temperatures. Post mortem tests showed that premature ageing could clearly be attributed to the low exhaust gas temperatures. As a result of the low exhaust gas temperatures, unburnt hydrocarbons and particles deposit on the catalytic coating, which reduces the catalytic activity of the coating. Thus, these tests show that reduction in catalytic activity of the catalyst is caused primarily by coverage of the catalytically active centres with soot particles that have hydrocarbons adsorbed thereon. Several such particles or combinations with particles of a different composition stick together and thus clog the pores and lower the active surface area of the catalyst. This results in an impairment of the oxidizing power of the catalyst.
Other components in the exhaust gas poison the catalyst by covering it or by reacting with constituents in the catalytic coating. For example, sulfur oxides react with constituents in the coating and form sulfates, and phosphorus compounds produce a glassy coating on the surface of the catalyst. As a result of these effects, the catalytic activity may be impaired so much that compliance with the emission limiting values required by law is difficult.
The present invention provides a process in which the catalytic activity of a catalyst used for treating diesel exhaust gases can at least partly be regenerated, and thereby increases the useful life of the catalyst.
The present invention provides a process for regenerating the catalytic activity of a catalyst that has oxidizing functions. The catalyst is located in the exhaust gas line of a diesel engine that contains a catalytically active coating having at least one oxidizing function on a honeycomb carrier that does not have a filter function. In the process of the present invention, as a result of time-restricted increases in the exhaust gas temperature upstream of the catalyst to a value greater than 450xc2x0 C., the combustion of soot particles and hydrocarbons deposited on the catalyst is initiated, and thus, the catalytic activity of the catalyst is at least partly regenerated.
Examples of catalysts with which the process of the present invention may be used, include, but are not limited to, diesel oxidation catalysts, SCR catalysts, hydrolysis catalysts, HC-DeNOx catalysts and four-way catalysts.